Lab 4 How do Faults Work_spring 2024
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Baylor University *
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1401
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Geology
Date
Apr 3, 2024
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Laboratory 4: How do Faults Work? Before Attending Lab This Week... ...read this introductory material
.
Purpose of This Lab Have you ever felt an earthquake? Do you know how they are caused? Rock in the upper crust tends to deform elastically
. Elastic deformation is like stretching a piece of rubber tubing; once you stop stretching it, it returns to its original shape. Rocks deform elastically up to the elastic limit
. The
n
if more stress is applied, the rock breaks. Once an initial fracture is formed, further deformation occurs by frictional slip between intact rock masses, forming faults
.
Faults
are fracture surfaces in rock across which there has been displacement. The rock masses to either side of the fault surface are called fault blocks
(Figure 4-1). Rocks on the move Faults form when local stress conditions in Earth exceed the brittle strength of the rocks
, releasing the stored elastic energy that has been built up.
Depending on the properties of the rock, the release of
energy during fault motion can be violent, producing earthquakes
or peaceful
and not produce earthquakes.
The faults that produce earthquakes, like the San Andreas, are in hard rocks that can store lots of elastic energy before failure. This style of fault motion is called stick-slip
motion. The fault “sticks” during periods between earthquakes and then “slips” during earthquakes. Faults occur at all spatial scales. Some are tiny little breaks with only a few millimeters of offset that begin and end over a span of a few centimeters. Other faults, such as the San Andreas, extend for hundreds of kilometers and can have equally large offsets. Long faults pass through rock with different properties and with different levels of stress. As a result, it is common for long faults to exhibit different behaviors over different intervals along their lengths. Intervals along which a fault exhibits the same behavior are called segments of the fault. Lab 4: How do Faults Work? Figure 4-1. Fault indicated by arrow with displaced rock layer (photo from Marli Miller).
What type of fault is that? The sub-discipline of geology that deals with faults and other forms of crustal deformation is called structural geology
. As with all branches of science, structural geology has its own extensive vocabulary of specialized terms. The classification of faults is based on the geometry and relative motion of the adjacent fault blocks. Fault blocks are named according to their position relative to the fault surface. The foot-wall
block
is the block the lies beneath the fault surface and the hanging-
wall block
is the block that lies on top of the fault surface
(Figure 4-2)
There are two basic types of fault
s
: vertical faults and horizontal faults. Vertical faults are divided by the relative motion of the hanging-wall and foot-wall blocks.
For normal faults
, the hanging-wall block slides down
relative to
the foot-wall block. Figure 4-1 is a normal fault.
For r
everse faults
, the hanging-wall block moves up relative to the foot-wall block. The second main class of faults is strike-slip
faults. In strike-slip faulting, the relative motion of the fault blocks is horizontal.
Strike-slip faults are further divided into right-lateral
and left-lateral
strike-slip faults, depending on the sense of motion of the two fault blocks. Look across the fault to determine left or right lateral motion.
Figure 4-
3
. Strike-slip faults. How Can We Study Faults? Geologists study faults and the faulting process in a variety of ways. The most direct approach is to go look at faults in nature, where erosion or construction has exposed the fault surfaces. This approach tells geologists a lot about fault geometries, but unless they happen to be there during an earthquake, it tells them little about the faulting process. Geologists study how faults work using different kinds of scale models. This is the approach we will use in this lab. Lab 4: How do Faults Work? Figure 4-2. Normal and Reverse faults. Notice where the hanging wall and foot wall are compared to the fault surface. Terms to know
Fault Fault segment Foot wall Hanging wall Normal fault Reverse fault San Andreas
Fault Stick-slip Strike slip Structural geology
Group Project 4-1: How do Faults Relate to Plate Tectonics? Most active faults on Earth occur at the boundaries between tectonic plates. The relative motion between the plates causes the faulting. Furthermore, each type of plate boundary (divergent, convergent, and transform) is associated with a particular type of faulting. In this project we use a scale model of relative plate motion to see how different types of faults form on different types of plate boundaries. Task 1: Model of relative plate motion. At this station you will find a model that simulates faulting at different plate boundaries. The model consists of an outer box containing sand and an inner block of wood that can slide relative to the outer box. The inner block represents a tectonic plate on the surface of Earth. The sand-
filled region between the inner block and the outer box represents the zone of deformation around the boundary of the plate. The outer edge of the box represents the boundaries of the field of plates surrounding the inner plate. The actual surrounding plates are not represented in the model. To prepare the model for the experiment, use the tool provided to smooth the sand region between the inner block and the edge of the box. A light dusting of flour is used to highlight the model faults. From time to time the dusting of flour may need to be replenished. Task 2: Simulate relative plate motion. To simulate the relative motion of the inner plate relative to the surrounding plates, slowly push the inner block in the direction indicated, relative to the outer box. The resulting relative motion produces a simulated convergent boundary in front of the leading edge of the inner block, in the direction of motion. A divergent boundary is simulated behind the trailing edge of the block. Transform boundaries are simulated on either edge of the inner block that are parallel to the motion. Carefully observe how the sand deforms on the different edges of the model plate as it moves. Record your observations and answer the associated questions on the answer sheet. When you are done, slide the inner block back to its starting position and re-smooth the sand for the next group. Lab 4: How do Faults Work?
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Name: ___________________________ ID: ___________________________Group:________ Answer Sheet 4-1:
How do Faults Relate to Plate Tectonics? 1.
Describe what kind of deformation occurs along the leading edge (in front movement direction)
of the inner block as it moves. If you see faults forming, what kind of faults are they (normal,
reverse, left-lateral strike slip, or right-lateral strike slip)?
_____________________________________________________________________________ _____________________________________________________________________________ 2.
Describe what kind of deformation occurs along the trailing edge (away from the movement
direction) of the inner block as it moves. If you see faults forming, what kind of faults are they
(normal, reverse, left-lateral strike slip, or right-lateral strike slip)?
_____________________________________________________________________________ _____________________________________________________________________________ 3.
Describe what kind of deformation occurs along the side edges (parallel to the plate motion) of
the inner block as it moves. If you see faults forming, what kind of faults are they (normal,
reverse, left-lateral strike slip, or right-lateral strike slip)?
_____________________________________________________________________________ _____________________________________________________________________________ Questions: 4.
The Himalayan Mountains mark a plate boundary between India and Asia where the crust is
being pushed up as India pushes into Asia. From the results of this project, what plate boundary
type would this be (divergent, convergent, or transform)?
_____________________________________________________________________________ 5.
A plate boundary runs the length of the California coast from San Diego to north of San
Francisco the results from the slip of the Pacific plate parallel to the coast. From the results of
this project, what plate boundary type is the San Andreas Fault (divergent, convergent, or
transform)?
_____________________________________________________________________________ 6.
Iceland sets astride the Mid-Atlantic Ridge, which has a deep valley with faults to either side
along the crest of the ridge. From the results of this project, what boundary type is the Mid
Atlantic Ridge (divergent, convergent, or transform)?
Lab 4: How do Faults Work? _____________________________________________________________________________
Group Project 4-2:
How Do Strike-Slip Faults Work? Strike-slip faults involve the horizontal movement of fault blocks. Although the idea of two rock blocks slipping past one another seems simple, strike-slip faulting results in complex patterns of rock deformation. In this project we examine how strike-slip faults form using scale models in which powder is used to represent rock. Task 1: Simulation of simple strike-slip faulting. At this station you will find two models for simulating strike-slip faulting. For this task use the box for which the sides are of equal length and symmetric. In nature, large faults form as smaller faults grow and coalesce (link together). This box simulates the formation of a straight strike-slip fault. Pure flour is used to make the initial cracks visible. To prepare the box, orient the model as shown above, with the sliding half of the box furthest away from you. Align the two sides of the box to form a rectangle and initially smooth the powder with the provided scraper. Do this carefully, trying not to spill the powder on the table or to cause it to fly into the air. Then finish the smoothing process by tamping the powder with the wood block. To simulate left-lateral strike-slip motion, slide the movable
part of the box slowly to the left, about one-half inch at a time, relative to the fixed side of the box. As you move the sliding box, notice the main
fault that form
s
in the powder. In your drawing, indicate the sense of motion of each side of the developing main fault with arrows. Now slide the box-side back to the starting position, aligned with the fixed side, and re-
smooth the powder. Repeat the experiment described above but this time simulate right-lateral strike-slip motion by sliding the movable
side of the box to the right, relative to the fixed side. Again, stop when the main
fault is
clearly visible and draw the fault pattern and sense of motion in the space provided on the answer sheet. Answer the related questions. Lab 4: How do Faults Work?
Lab 4: How do Faults Work? Task 2: Simulation of strike-slip faulting with restraining and releasing bends. In the second model at this station, the box has different length sides on either end to accommodate a bend and offset in the fault. When viewed from either end of the model, the bend and offset are to the right. This type of bend is appropriately called a right-bend in the strike slip fault. Prepare the model as before, by aligning the sides and smoothing the powder. In this model a mixture of 50-50 fine sand and flour is used to produce more realistic fault angles. Simulate left-lateral strike slip motion around the right bend by sliding the moveable box side slowly to the left, about one-half inch at a time, relative to the fixed box side.
Continue the motion until the sliding box side reaches the stop, while watching how the region adjacent to the fault bend deforms. Draw the resulting fault pattern in the space provided. Indicate the sense of motion of each side of the main fault with arrows.
Once your drawing of left-lateral motion is complete, re-align the box sides and smooth the powder. Then simulate right-lateral strike-slip motion around a right bend by slowly sliding the moveable side of the box to the right, relative to the fixed side of the box. When the sliding box-side has reached the stop, draw the resulting fault pattern in the space provided on the answer sheet. Mark uplifted regions of the powder surface with plus signs, “+” and down-dropped regions with minus signs, “-“. Answer related questions on the answer sheet.
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Name: ___________________________ ID: ___________________________Group_______ Answer Sheet 4-2:
How Do Strike-Slip Faults Work? Task 1: 1.
Draw the fault pattern associated with the development of a simple left-lateral strike-slip fault:
2.
Draw the fault pattern associated with the development of a simple right-lateral strike-slip fault:
3.
Is the orientation of the main fault the same no matter if the box was moved left or right?
_________________________________________________________________________
4.
What clues could you use to determine whether a strike slip fault was left-lateral or right
lateral if you weren't there when the earthquake occurred?
___________________________________________________________________________ ____________________________________________________________________________ Lab 4: How do Faults Work?
Name: ___________________________ ID: ___________________________Group:________ Answer Sheet 4-2 continued:
How Do Strike-Slip Faults Work? Task 2: 5.
Draw the faulting pattern associated with left-lateral strike-slip faulting around a right bend.
6.
Draw the fault pattern associated with right-lateral strike-slip faulting around a right bend.
7.
Given that rocks are strongest in compression (squeezing together) and weakest in tension
(pulling apart), which of these two fault motions, left-lateral or right-lateral, will require greater
force for the fault block to move around the right bend?
___________________________________________________________________________ 8.
Near Los Angeles there is a pronounced bend in the San Andreas Fault. The San Gabriel
Mountains have formed at this bend in response to motion on the San Andreas. Which of the two
fault patterns in Task 2 do you think is analogous to the San Andreas adjacent to the San Gabriel
Mountains?
___________________________________________________________________________ Lab 4: How do Faults Work?
Group Project 4-3:
How do Vertical
Faults Work? Vertical faults move in response to either compressional or extensional tectonic forces. The faults are classified based on the motion of the fault block above the fault (hanging wall) relative to the fault block below the fault (foot wall). In normal faulting, the hanging wall moves down relative to the foot wall. In reverse faulting, the hanging wall moves up
relative to the foot wall. In this project we will generate both reverse and normal
faults using scale models. Our goal is to see how fault motion is related to the driving force (compression or extension)
.
Task 1
: Simulation of Fault Development in Extension.
At this station you will find two box models for studying the development of vertical
faults. The two models are the same, so either one can be used for this task. Prepare one of the models by dumping any leftover sand in the model into a bin of mixed sand and turning the drive screw to bring the moveable wall in alignment to the Initial Position Mark on the base of the model. Then use the provided sco
o
p and wood paddle to fill the model with 2 alternating layers of 1/4 inch thick w
hite sand
/colored chalk
. Simulate fault development in extension, by turning the hand crank counter-clockwise. As faults form, they will break the sand and chalk
layers. Look from the side, through the Plexiglas, for offsets in the original smooth layers. Continue until at least one fault with significant offset has formed. Draw the fault pattern you see in the space provided on the answer sheet. Lab 4: How do Faults Work? 4-
9
Task 2
: Simulation of Fault Development in Compression. Prepare the second model for use, as before, by dumping the contents into the bin for mixed sand and chalk
, aligning the moveable wall with the initial position mark, and filling the box with 2 alternating layers of 1/4 inch thick white sand
/colored chalk
. Simulate fault development in compression, by turning the drive screw clockwise. Continue until at least one fault with significant offset has formed. Draw the fault pattern you see in the space provided on the answer sheet.
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Name: ___________________________ ID: ___________________________Group:________ Answer Sheet 4-3:
How do Vertical
Faults Work? Extensional fault pattern: Compression
al
fault pattern: 1.
What type of fault is produced by extensional tectonic forces?
_____________________________________________________________________________ 2.
What type of fault is produced by compressional tectonic forces?
____________________________________________________________________________
3. Discuss the likelihood of of faults developing if the layers of sand and chalk were thicker.
____________________________________________________________________________
4. Discuss the likelihood of faults developing if there were more layers of sand and chalk.
____________________________________________________________________________
Lab 4: How do Faults Work?
Group Project 4-4:
How does Stick-Slip Fault Motion Work?
In this project we use models with foam rubber and springs to simulate the stick-slip style fault motion that produces earthquakes. Step 1: Stick-slip simulation with frictional sliding and a spring. At this station you will find a model of stick-slip fault motion. For this step use the hand-crank model with just one top block and spring. Place a single small block on top of the long bottom block with the foam sides facing each other and attach the spring to the stand as shown below: In this model, the bottom block represents one side of a fault, which is being moved by tectonic forces relative to the top block. The small top block represents the opposite side of the fault. The foam rubber between the blocks is used to provide friction on the model fault surface. The spring attached to the top block represents the elastic properties of the rock. Because this model can store and release elastic energy, it is capable of producing short bursts motion across the fault, separated by periods of no relative motion. This type of motion is called “stick-slip”. The sudden movements of the top block present earthquakes. Periods in which the top block just rides along with the bottom block while the spring stretches represent time intervals between earthquakes. To operate the model, slide the long bottom block away from the hand winch. Then slowly and steadily crank the hand winch, pulling the bottom block towards the winch. The process is rate sensitive. Operate the model several times, at different speeds. Watch the motion of the top block as the spring stretches and releases. You are looking for stick-slip motion, which is characterized by stop-and-start movement of the top block. If the top block sticks fast to the bottom block and does not move, add a little sand to the top of the foam on the bottom block. It the top block slides without stopping, scrape a little bit of the sand off the bottom block. Record your observations and answers to the related questions on the answer sheet. Lab 4: How do Faults Work?
Step 2: Simulate stick-slip motion within multiple fault segments. The apparatus as used in Step 1 provides a simple model of stick-slip motion.
However, f
ault motion does not generally occur all along a fault at one time. Different segments of faults tend to move at different times. In this step we use a model with two upper blocks to generate a more realistic representation of stick-slip motion on faults with multiple segments. Slide the long bottom block away from the hand winch as before and place the second small block on top. Hook the spring on the second block to the tail end of first block as shown below: In this configuration different amounts of elastic energy can be stored in the springs between the upper fault blocks. This represents different amounts of elastic deformation of rock in different fault segments. Energy released from one fault segment can be transferred and stored as elastic energy in the adjacent segment. To operate the model, slowly and steadily crank the hand winch, pulling the bottom block towards the winch. The process is rate sensitive. Operate the model several times, at different speeds. Watch the motion of Block 1 relative to Block 2 as springs to either side stretch and release. Notice how the motion of Block 1 is related to the motion of Block 2. Look to see if they tend to move at the same time or if first one moves and then the other. Record your observations and answers to the related questions on the answer sheet. Lab 4: How do Faults Work?
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4-1
3
Lab 4: How do Faults Work? Name: ___________________________ ID:_____________________Group No:_________
Answer Sheet 4-4:
How does Stick-Slip Fault Motion Work? Step 1: 1.
Describe what happens to the top block as the bottom block moves at slow, steady speeds.
___________________________________________________________________________ ___________________________________________________________________________ 2.
Describe what happens to the spring as the bottom block moves at slow, steady speeds.
___________________________________________________________________________ ______________________________________________________________ 3.
Does the same thing happen at all times and all speeds?
Describe any differences you see.
___________________________________________________________________________ ______________________________________________________________ Step 2: 4.
Describe how the motion of Block 1 is related to the motion of Block 2.
__________________________________________________________________________ __________________________________________________________________________ 5.
Describe how the extension and release of the spring in front of Block 1 is related to extension
and release of the spring in front of Block 2.
__________________________________________________________________________ __________________________________________________________________________ 6.
Does the same thing happen at all times and all speeds
?
Describe any differences you see.
___________________________________________________________________________ ______________________________________________________________